Conventionally, for a camera's autofocus detection apparatus, an autofocusing apparatus which operates in the following manner is well-known. Light beams from an object that have passed through different exit pupil areas of a photographing lens are led to form an image on a pair of line sensors, and a relative positional displacement amount of a pair of image signals obtained from the line sensors is acquired, thereby detecting a defocus amount of the object and driving the focus lens in accordance with the defocus amount (e.g., refer to Japanese Patent Laid-Open No. 2000-292686).
Furthermore, there has been a proposed technique for realizing phase-difference focus detection by providing a phase-difference detection function to an image sensor and not requiring a dedicated AF sensor.
For instance, according to Japanese Patent Laid-Open No. 2000-156823, a pupil division function is provided to part of light receiving elements (pixels) of an image sensor by eccentrically having the sensitivity region of the light receiving area with respect to the optical axis of an on-chip microlens. By arranging these pixels serving as focus detection pixels at predetermined intervals between image sensing pixels, phase-difference focus detection is realized.
Meanwhile, in a charge-storage-type sensor, the charge storage amount itself reduces in a low-brightness object. Therefore, the S/N ratio of the sensor output declines, causing a problem of deteriorated focus detection precision.
In general, focus detection pixels disclosed in Japanese Patent Laid-Open No. 2000-156823 have a smaller opening portion than that of dedicated AF sensor pixels. Therefore, the S/N ratio for a low-brightness object declines. This is because of the following reason. Since the place where the focus detection pixels are arranged becomes a defective portion of the image sensing pixels, it is necessary that the size of the focus detection pixels be the same as or smaller than the size of the image sensing pixels. Therefore, the size of the focus detection pixels cannot be designed freely as in the dedicated AF sensor pixels.
In view of the above, Japanese Patent Laid-Open No. 10-319311 discloses a focus detection apparatus comprising pixel pitch switching means which can combine or separate the neighboring photoelectric conversion pixel arrays. This focus detection apparatus uses the pixel pitch switching means, when sensing a low-brightness object, to combine the neighboring photoelectric conversion pixel arrays, thereby increasing the charge storage amount and improving the S/N ratio.
Meanwhile, in phase-difference focus detection, it is known that an object's high-frequency component exerts a negative effect on detection precision.
In view of the above, Japanese Patent Laid-Open No. 6-308379 discloses a configuration for optically cutting a high-frequency component by slightly inclining the line sensor pixels with respect to the horizontal and vertical directions of the photographing screen.
However, according to the focus detection apparatus disclosed in Japanese Patent Laid-Open No. 10-319311, if image signals are formed by adding neighboring pixel signals in order to improve the S/N ratio, the pixel pitch becomes coarse and the resolving power of the spatial frequency that can be detected will reduce. For this reason, an error of the focus detection result obtained from high-frequency components of an object will become large.
Furthermore, the focus detection apparatus disclosed in Japanese Patent Laid-Open No. 6-308379 has a problem in that a high-frequency component cannot optically be cut if an object is in parallel with the inclined pixels and has a high frequency.